Fluorescent light bulbs or tubes are in widespread use and are commonly used in commercial and industrial applications. To operate a fluorescent bulb, a ballast is required that converts standard alternating current (AC) electrical service into an AC power signal at a magnitude and frequency higher than commercial AC service which causes ionization of the gas inside the fluorescent tube. While fluorescent tubes are more efficient than incandescent bulbs, they are not as efficient as LED lighting devices in terms of output light versus input electrical power. Accordingly, there has been interest in developing an LED lighting device as a replacement for fluorescent tubes.
One challenge in developing a replacement for fluorescent tubes is the fact that fluorescent tubes are powered by a ballast. The cost of rewiring a fluorescent lighting fixture to bypass a ballast is prohibitive, therefore it is desirable to make a “drop in” replacement that can be powered by an existing ballast. There are several different types of ballasts (i.e. electronic and magnetic) commonly in use, however, each having different electrical output characteristics for pre-heat (with starter), rapid start, program start, semi-resonant, and instant start type fluorescent bulb types. Accordingly, these output characteristic need to be addressed when designing a power conversion circuit for driving LEDs from the ballast. This has led to the development of different LED replacement tube devices for use with different ballast types due to the small area available inside a tube housing designed to house the LEDs and fit within a standardized fluorescent tube configuration. Conventional LED replacement tube lighting device are designed for a specific type of ballast and are not capable of proper operation with a different type of ballast. In some cases the use of LED replacement tubes involves the removal of the ballast, which can represent a significant cost.
Another issue is that fluorescent bulbs generally operate at one light output level because of their design. A threshold current is necessary to support the continued ionization process that results in light. This means that the variability of light output for a particular bulb is not particularly variable, as it is with an incandescent bulb, for example. To reduce light in a commercial or industrial setting some of the bulbs are simply turned off, leaving enough on to produce the desired lower light level. However, this not a suitable solution in applications where there are only one or a small number of tubes for a smaller space being lit. Even with a small number of tubes, turning one or more off can leave dark spots in an area being lit. In commercial or industrial setting the light fixated are typically mounted higher than in, for example, and office setting. The higher mounting allows light to diffuse over a larger are, so even when some bulbs are turned off, the problem of dark spots is mitigated. Conventional LED replacement lighting tube devices are designed in such a way that they output only one light level. Different models are made to output different light (i.e. lumen output) levels, which requires a manufacturer to design, manufacture, and stock different models for different applications and light output level, as well as by different types of ballasts.
Accordingly, there is a need for a LED lighting device that can be powered by any of the standard ballast types, and which can provide a variable light output level while remaining in the standard tube configuration.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. The details of well-known elements, structure, or processes that would be necessary to practice the embodiments, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated.